The org.openide.DataFolder.getChildrenList() is package 
private. When called from the 
org.openide.loaders.DataObjectFilter$4 it yields an 
IllegalAccessError. This might seem inappropriate since 
they actually reside in the same package.

My guess is that the offending class is loaded using a 
different classloader. From a security perspective, that's 
the same thing as residing in different package.

This is the exception detail:
Annotation: Exception occurred in Request Processor
java.lang.IllegalAccessError: tried to access method 
org.openide.loaders.DataFolder.getChildrenList()
Ljava/util/List; from class 
org.openide.loaders.DataObjectFilter$4
	at org.openide.loaders.DataObjectFilter$4.run
(DataObjectFilter.java:409)
	at org.openide.util.Task.run(Task.java:136)
	at org.openide.util.RequestProcessor$Task.run
(RequestProcessor.java:328)
[catch] at org.openide.util.RequestProcessor$Processor.run
(RequestProcessor.java:667)